I received a call from a greenhouse grower recently with a tomato transplant problem. The grower described yellow seed leaves and curled foliage. The grower was able send a couple excellent photos (Figure 1). There were a few clues that the symptoms might be due to a heater problem. In Figure 1, some of the seedlings have leaves that are curled down and stems that are twisted (epinasty in botanical terms). Epinasty is a common symptom of ethylene damage. Ethylene is a common by-product of incomplete combustion of several different types of fuel. Incomplete combustion is often the result of heaters that are not working efficiently. Tomatoes are very sensitive to ethylene damage.
A second clue is to take a closer look at the yellow seed leaves (Figure 1). Ethylene damage does not include yellowing. Furthermore, there is a spotting on the lower leaves that is not typical ethylene damage. I believe that the symptoms on seed leaves were as a result of a different compound, perhaps sulfur, a heavier than air compound that would remain relatively close to the heater. The grower confirmed that the yellowing leaves were close to the heater, while the curling leaves, caused by ethylene gas, were spread throughout the greenhouse.
While some greenhouses are heated with a furnace attached to the greenhouse, many greenhouses are heated with a standalone unit inside the structure. In the example above, the grower stated that the heater was of this latter type-a standalone unvented unit. While this type of heating is not recommended, natural gas, propane and kerosene generally burn clean and do not need to be vented. However, even units that burn clean fuels may cause problems if out of adjustment (see citation below).
I cannot prove that the symptoms in Figures 1 above are caused by ethylene. But a few years ago, we witnessed ethylene-like damage at a greenhouse here at the Southwest Purdue Agriculture Center (See article in the November 2007 Vegetable Crops Hotline. Therefore, we were able to confirm that ethylene was the cause of the symptoms shown in Figure 2. Given the similarities of the two examples and the circumstantial evidence, I believe the example given in Figure 1 was due to a heater malfunction. The grower reports that after the heater was serviced, the plants began to look healthier.
Poorly adjusted heaters can also add water to the greenhouse air-as much as 22 gallons of water a night! This unwanted moisture can lead to disease problems.
To avoid damage from ethylene and other air pollutants:
1. Have unit heaters checked by a professional and follow maintenance recommendations.
2. Assure adequate air supply for complete combustion. For each 2500 BTU’s of heater output, 1 sq. in. of vent cross section is needed.
3. Prevent back drafts. Make sure the chimney extends 2 ft. above the ridge of the greenhouse, or 2 ft. above a 10-ft. line to any part of the structure.
4. Install an inexpensive carbon monoxide detector. If carbon monoxide levels rise it’s likely ethylene and other pollutants are present also. And if carbon monoxide levels are high it is a significant human health hazard.
5. Scout for possible growth effects of ethylene and investigate right away if you see anything.
Additional Resources: Bartok, J.W. Problems With Using Unvented Greenhouse Heaters
Figure 1: The tomato seedlings above exhibit downward curled
leaves (red arrows) which maybe a symptom of ethylene
damage and yellow seed leaves with lesions (red circles),
a possible symptoms of sulfur damage (Contributed Photo).
(Click image for larger view)
Figure 2: These tomato plants are exhibiting epinasty or a downward
growth of the leaves in response to ethylene produced from
a malfunctioning heater in a greenhouse. The topmost leaves
are growing normally because the plants were removed to
a separate greenhouse after exposure to ethylene.
(Photo by Dan Egel). (Click image for larger view)